Fix a potential NULL pointer dereference in free_shared_mem() in drivers/net/s2io.c

This patch fixes a potential null dereference bug where we dereference
nic before a null check. This patch simply moves the dereferencing
after the null check.

Signed-off-by: Micah Gruber < micah.gruber@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>